How to Make a Pre-Thickened Silicone Brush-On Mould

June 26th, 2010 |

There are times when it is essential to use a pre-thickened type of rubber using a brush-on application to make a mould of an object. An example of this would be to reproduce architectural ornamentation attached to a wall that could not be removed. Another would be the duplication of large pieces of sculpture. It would be difficult to apply the regular type of silicone or polyurethane used to make moulds by pouring, to do this type of vertical application without them sagging, so a pre-thickened type of silicone such as Gelsil is used.

1. Making Good the Model Surface: Before commencing the mould making, make good the surface of the original by repairing any cracks or damaged portions and allow the repairs to dry.

2. Some Models Must Be Sealed: The next step is to seal the surface of all porous articles, such as plaster, concrete, wood or stone. One or two coats of fast drying shellac may be sprayed on using the aerosol form of shellac available at most art or hardware stores, as this does not interfere with the surface detail. Spraying the sealer has the advantage that the coverage of rough detail and undercuts is easy. If sealing is not done the silicone will penetrate into the interstices of the porous surface and would be difficult or impossible to remove.

Sculptors please note, shellac is also suitable for sealing modelling clays that contain sulphur such as Roma Plastelina and clays containing moisture. It is not necessary to seal objects made of glass, glazed ceramic, sulphur free clays and hard plastics such as acrylic and PVC.

In all cases the sealing agent must be allowed to dry thoroughly before continuing on to the next step.

3. Application of the Release Agent: The release agent should now be applied. The aerosol Stoner 234 is recommended for silicones and should be lightly sprayed across all surfaces that will be in contact with the rubber and allowed to dry. Another release agent that can be used is a 10% solution of Vaseline or petroleum jelly dissolved in naptha or white spirit. When using most silicones this step is often not essential but until you have tested the surface it is better to be sure than sorry. For all polyurethane rubber, followed by a light spray, as polyurethane sticks to anything.

Before starting any major project, the effectiveness of the sealer/release combination should be tested using similar surfaces, on a small scale before starting the main work.

4. Measuring and Mixing: This will require two plastic containers as they must be weighed. Gelsil requires 10 parts by weight of the catalyst to 100 parts of the silicone. The pot life is 20 minutes and the model can be de-moulded in 6 to 12 hours. We usually leave overnight. It is Thixotropic by nature and is ideal for brush on application. It has a shore hardness of 25 and is capable of picking up very small detail. It has a low linear shrinkage of 0.40% and high tear strength. It is very suitable for reproducing models with pronounced undercuts.

Mixing tools should be of metal, glass or plastic. The Gelsil catalyst is coloured to ensure complete mixing. After weighing out the required quantities of silicone and catalyst pour the catalyst into the silicone container and mix thoroughly for 3 minutes making sure you scrape the sides and bottom of the mixing container several times. Use a flat paddle action rather than a brisk beating action to reduce the amount of air you beat into the mixture. You will know when you have achieved a good mix when you have a good even colour.

5. Rubber Application: The silicone must be applied in layers. The number of layers of rubber required is between 4 to 6 dictated by the size and shape of the model. Use one of the cheap Chinese white pigs bristle brushes sold in most art shops for brushing on the silicone. The fist coat is important as this will pick up all the fine detail of the model. Apply this “detail coat” in a thin layer taking particular care to cover the entire surface. Use a dabbing action rather than a painting action especially around undercuts to reduce the amount of entrapped air and to pick up all detail. Let this first coat dry till it becomes tacky before adding the next coat.

Repeat adding layers of silicone in thin layers especially for the first 3 or 4 coats till the necessary thickness is achieved. Later layers can be applied thicker providing the silicone is not applied so thickly it drips off the model. To prevent the possibility of de-lamination do not allow any of the layers to fully cure but apply each new coat when the previous coat has become “tacky”. When working in difficult or dark conditions, a mould maker’s trick is to add colour to each layer to ensure complete coverage has been achieved, but this is not essential in normal working conditions.

6. Curing: Allow the mould to cure at least, overnight before making the back-up mould. In cold conditions leave for at least 16 hours to ensure complete curing. After the back-up mould is made and the silicone mould is removed post curing of the mould for 4 to 6 hours at 60 degrees celsius is an advantage and will increase the physical properties a great deal.

7. Applying a Back-Up Mould: To ensure subsequent castings are accurate the silicone mould must be held in the correct shape by a support or back-up mould. This can be made of either plaster or fibreglass. For the plaster back-up use an oil soap release agent applied to the exterior of the silicone mould. For the fibreglass use PVA release agent.

It is essential to locate any undercuts on the silicone mould that would prevent its easy removal and fill them in with Plastelina Clay or alternatively make a two piece back-up mould.

8. Using the Silicone Mould: If the mould is to be used for concrete use a vegetable oil thinly applied as a release agent. For plaster use an oil soap release. This should be applied to the mould before each casting and any excess accumulating in undercuts or detail should be removed with a cloth to remove pooling of the release agent.

9. Mould Life and Storage: When fully cured these moulds are tough and have a high tear strength if properly used and stored. The life depends on how you use it. Abrasive materials such as concrete will eventually cause mould detail to deteriorate while non abrasive material will not affect detail. After use wash moulds with warm soap and water and let dry. Moulds should be stored on a level surface in a dry non sunny, cool environment.

Stan Alderson.

Important: The Material Safety Data Sheets (MSDS) should be read prior to using any Industrial moulding making product and are available upon request from Aldax or may be viewed on their website. Directions should be read and followed carefully to ensure safety.

The information contained in this article is considered accurate. However, no warranty is expressed or implied regarding the accuracy of the data, the results to be obtained from the use thereof, or that any such use will not infringe upon a patent. User shall determine the suitability of the product for the intended application and assume all risk and liability whatsoever in connection therewith.

For Technical help:

Tel. 02/9533 9555

Fax 02/9533 9955

Website: www.aldax.com.au